This invention relates to the field of touch-controlled switches and to the field of burglar alarms.
Prior art touch switches tend to have features that make them unsuitable for use in alarm systems, or they lack features necessary to make practical alarm systems.
Many touch-controlled circuits depend on AC power lines to provide the oscillating voltage required for capacitively operated switches. This dependency on AC power lines makes these circuits unsuitable for use in alarm systems which are required to operate during power failures.
Some touch-controlled switches create the required oscillating voltage with an internal oscillator, and thus could be adapted to operate from battery power during a power failure. Examples of touch switches using internal oscillators are U.S. Pat. Nos. 4,668,877 to Kunen (1987), and 4,668,876 to Skarman (1987). These switches have a significant drawback, however. They both have an oscillator which operates at a radio frequency, thus making them subject to governmental regulations regarding radio interference. Another common problem with oscillator-based switches is the need to manually adjust the parameters of components during the manufacturing process. U.S. Pat. No. 4,550,310 to Yamaguchi et al. (1985), for example, has tuned resonant circuits which must be precisely adjusted.
The trigger circuits of prior art touch switches are unidirectional, thus requiring two trigger circuits if both increases and decreases of capacitance are to be detected. U.S. Pat. No. 4,081,700, to Hamilton, II (1978), for example, suggests using two complementary trigger circuits.
Another feature desirable for alarm systems but unavailable in the prior art is a touch switch that is automatically triggered by disconnecting its power plug, but is not triggered by AC power fluctuations or failures.